Remarkable progress has been made in liquid crystal displays and the application of the liquid crystal displays is expanding not only to small- and medium-sized monitors for mobile phones and personal computers but also to large-sized TV screens. A polarizing plate is used in the liquid crystal displays, and a laminated film prepared by sandwiching both sides of a polarizing film obtained by impregnating a polyvinyl alcohol film with iodide or a dichroic pigment and stretching the film between triacetyl cellulose (to be abbreviated as “TAC” hereinafter) films has been commonly used as the polarizing plate.
Although TAC used as a protective film for the polarizing film has advantages such as excellent adhesion to the polarizing film, high transparency and a good appearance, a polarizing plate having a TAC film as a protective film has low durability in a high-temperature high-humidity environment because TAC has high water absorption and water vapor permeability. The TAC film has various disadvantages such as insufficient physical strength with a low tensile strength of about 6 to 11 kg/mm2.
The TAC film is generally manufactured by a solvent cast method. Although the retardation R in the in-plane direction of the film can be made low by the production process in these film forming methods, it is known that the plane orientation of the polymer tends to be high and therefore, it is difficult to make the retardation Rth in the thickness direction low. It is described in the “Development of Low-Retardation TAC Film” of FMC11-2, p. 1317 of the collection of lectures at the 12-th IDW/AD'05 that the high Rth of a TAC film is not desirable for application in IPS (In-Plane-Switching) mode liquid crystal displays.
Films made of various materials have been studied as a protective film for polarizing plates in place of the TAC film. For example, Japanese Patent No. 3297450 reports the use of a thermoplastic saturated norbornene-based resin in place of the TAC film. Although the thermoplastic saturated norbornene-based resin has excellent optical properties, it has low adhesion to a polarizing film. Further, as it has extremely low moisture absorptivity and water vapor permeability, after it is laminated with a polarizing film, water contained in the polarizing film is hardly removed. An unstretched film having low retardations is used as a protective film for polarizing plates. Since the resin has low toughness, its unstretched film has low mechanical strength.
Meanwhile, the use of a polycarbonate resin as a protective film for polarizing plates is now under study. The polycarbonate has appropriate moisture absorptivity and water vapor permeability and its unstretched film has high mechanical strength. For example, JP-A 8-62419 reports a protective film for the polarizing film which is a polycarbonate resin film obtained by the solvent cast method. However, the polycarbonate resin film manufactured by the solvent cast method has a defect which will be described hereinafter.
In the field of optical discs, a polycarbonate resin has been used from a long time ago. Various researches, development and commercialization of optical discs as high-density large-capacity recording media have been made, and various technologies that enable a large volume of data including video information to be recorded on an optical disc have been developed. For instance, a film plane incident type optical disc technology for reading information from one side of an optical disc is proposed in JP-A 08-235638, “Large-volume optical disc having a recording capacity of 12 Gbytes on one side”, OplusE, vol. 20, No. 2, p. 183 (February 1998) and “Optical Discs and Peripheral Materials”, 98-2 polymer photoelectronics study meeting lecture collection, the Polymer Electronics Study Meeting of the Polymer Association (Jan. 22, 1999). The film plane incident type optical disc is a high-density recording medium which is mainly made from a polycarbonate resin and has a disc-like information recording layer. The information recording layer is formed on the surface of a disc having a thickness of about 0.6 to 1.1 mm. To protect this information recording layer, a film having a thickness of about 0.01 to 0.1 mm is adhered (bonded) to the information recording layer by an adhesive or the like. This protective layer is called “light transmitting layer” among people of ordinary skill in the art.
As the requirements for the light transmitting layer, it must have high transparency, uniform thickness, a low in-plane retardation and a low retardation in the thickness direction and almost the same mechanical properties, especial thermal shrinkage factor as that of a polycarbonate as the main material of an optical disc substrate.
As means of manufacturing a polycarbonate resin film, there have been known a solvent cast method and a melt extrusion method. Out of these, the solvent cast method has been mainly studied for the optical application of the polycarbonate resin film.
For example, the above JP-A 8-62419 relates to a protective film for polarizing films which is a polycarbonate resin film manufactured by a solvent cast method or solvent cast method.
JP-A 2002-074749 discloses that a resin sheet manufactured by a melt extrusion method (substantially the solvent cast method) is used as a light transmitting layer for optical discs and a polycarbonate may be used as one type of the resin. JP-A 2001-243658 and JP-A 2001-243659 disclose a process of manufacturing a light transmitting layer from a polycarbonate by the solvent cast method. Further, a polycarbonate resin film manufactured by the solvent cast method is used in the currently available “Blu-ray Disc” (trade name).
However, the solvent cast method has a cost problem and also a problem that when a thick film is to be manufactured by the solvent cast method, a transparent flexible film is hardly obtained as crystallization occurs in the step of removing the solvent after the solution is applied. The latter problem is an obstacle to the further increase of the density of an optical disc. That is, in a high-density optical disc having two information recording layers, two light transmitting layers having a thickness of about 50 μm and a thickness of about 100 μm are used. The light transmitting layer having a thickness of about 50 μm is positioned in the inside of the optical disc and the light transmitting layer having a thickness of about 100 μm is positioned on the surface layer of the optical disc. The polycarbonate resin film having a thickness of about 100 μm is hardly manufactured by the solvent cast method.
When a film is to be manufactured from a methylene chloride solution of a polycarbonate of the same type as that used as the main material of an optical disc substrate, that is, a bisphenol A-polycarbonate having a relatively low molecular weight of about 15,000 in terms of viscosity average molecular weight and the thickness of the film is about 50 μm or more, the crystallization of the polycarbonate readily occurs in the step of evaporating the solvent, thereby making it extremely difficult to obtain a transparent film. Therefore, to manufacture a light transmitting layer film for optical discs by the solvent cast method, a polycarbonate resin having a high molecular weight (viscosity average molecular weight of about 35,000 to 40,000) which hardly crystallizes is used. However, as an optical disc substrate obtained by injection molding a polycarbonate resin having a low molecular weight and a high-molecular weight polycarbonate resin film manufactured by the solvent cast method differ from each other in physical properties, a disc obtained by laminating them together has a problem with quality, especially long-term stability.
Further, a polycarbonate resin film obtained by the solvent cast method has highly in-plane orientation which is a property not suitable for use as a protective film for polarizing films or light transmitting layer for optical discs.
Then, attempts are being made to manufacture a polycarbonate resin film for optical use by the melt extrusion method.
For example, Japanese Patent No. 2932731 (JP-A 4-275129) discloses that a polycarbonate resin film having reduced birefringence at a thickness of 0.2 to 2.0 mm can be manufactured under melt extrusion film forming conditions: (i) a resin temperature of 300 to 330° C., (ii) an air gap of 80 to 100 mm and (iii) a chill roll temperature of 100 to 140° C. Japanese Patent No. 3417696 (JP-A 8-171001) discloses that both end portions of a film are closely contacted and/or pressure contacted to a cooling drum as a process of manufacturing a polycarbonate resin film for use as an electrode substrate for liquid crystal display panels. However, these melt extrusion film forming methods have a defect that the thickness of the formed film changes like undulation and inconvenience that fine streaks are gradually formed on the surface of the film when film formation is continued for a long time.
Meanwhile, JP-A 60-214922 discloses a process of manufacturing an optically non-rotatory thermoplastic resin film by extrusion molding an amorphous thermoplastic resin while applying static electricity to the film to closely contact and fix it on a chill roll. The above publication also discloses that when this technology is carried out, an electrostatic wire is used as an electrode. JP-A 4-166319 proposes a method of manufacturing a sheet having a thickness of about 0.1 to 3 mm and a birefringence in the film width direction of 40 nm or less by melt extruding a polycarbonate resin having a viscosity average molecular weight of 14,000 to 19,000. Further, JP-A 10-217313 discloses a method of manufacturing a polycarbonate resin sheet for optical use which is made of a polycarbonate resin having a viscosity average molecular weight of 14,000 to 19,000 and has a birefringence of 20 nm or less, a curl of 0.5% or less and a thickness of 0.1 to 1 mm. However, the polycarbonate resin films manufactured by these technologies have a defect that the birefringence of the both end portions in the width direction of the film is larger than that of the center portion. Therefore, it is impossible to stably manufacture a film having such a low birefringence uniformly in the width direction that enables it to be used as a polarizing plate protective film for liquid crystal display devices and a light transmitting layer for optical discs.
Further, JP-A 2003-302522 discloses a method of reducing the retardation to 10 nm or less by heating a polycarbonate resin film while tension is applied in the processing direction of the film after the film is manufactured by the melt extrusion method. Since this method requires a heat treatment after film formation, it is inferior in productivity disadvantageously. According to this method, the in-plane retardation can be made low but the retardation in the thickness direction of the film tends to become high. JP-A 2004-315636 discloses a polycarbonate-based resin film which has a birefringence in the plane of the film of 8×10−5 to 30×10−5 and an optical axis in the width direction of the film. However, the in-plane retardation R actually reported in Examples is not sufficiently low.
As described above, in the conventionally known processes of manufacturing a polycarbonate resin film, a film having a low in-plane retardation and a low retardation in the thickness direction as well as small thickness nonuniformity is not obtained yet, and this film and a technology for manufacturing this film by the industrially advantageous melt extrusion method are desired in the field of optical films, especially protective film for the polarizing films for liquid crystal display devices and light transmitting layers for optical discs.